I would not worry too much about the rear axle bearings. They are actually pretty durable, as most last 50 years or more. Easy to replace if needed.
If you have the rear end cover off to change the fluid, pull the circlips (E clips) off the axle ends, slide them out, replace the needle bearing and the oil seals on the end of each axle housing. Put the axles back in and replace the clips, button things up. You're good to go for another 50 years.
With an extra 500 lbs hanging off the back of the tractor, the bearings might only last 20 years...
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